1) Field of the Invention
The present invention relates to a process for producing pyromellitic acid by liquid phase oxidation of durene and oxidation derivatives thereof, and specifically to a process for producing pyromellitic acid which comprises oxidizing durene as a starting material in a liquid phase and producing intermediate products step by step and oxidizing thereof, thereby producing pyromellitic acid of a final product in a high yield.
2) Prior Art
Terephthalic acid has been industrially produced in many states by air oxidation of p-xylene in acetic acid solvent in the presence of a bromine-transition metal catalyst. In a liquid phase oxidation reaction employing aromatic hydrocarbons as starting material, it is indispensable to use acetic acid as a solvent in order to obtain aromatic polycarboxylic acids. When acetic acid is used as such solvent, loss of acetic acid occurs by its combustion.
Durene is oxidized with air in the presence of a heavy metal catalyst in the same manner as other alkyl aromatic compounds, whereby pyromellitic acid is produced. However, it is known that pyromellitic acid thus produced forms a complex with a heavy metal(s) due to ortho structure of two carboxylic groups in pyromellitic acid to cause deactivation of the catalyst, so that its yield is lower than that in other alkyl aromatic compounds having no such structure.
As a conventional process for producing pyromellitic acid, Japanese Patent Kokai (Laid-open) No. 57-38745 discloses a process for producing pyromellitic acid which comprises oxidizing polyalkyl aromatic aldehyde in acetic acid solvent in the presence of a cobalt/manganese/bromine catalyst.
Further, Japanese Patent Kokai (Laid-open) No. 2-184652 discloses a process for producing pyromellitic acid which comprises oxidizing durene in a liquid phase in the presence of a cobalt/manganese/bromine catalyst, wherein the catalyst is added in two stages and the reaction is performed in batch wise.
Japanese Patent Publication No. 7-116097 discloses a process for producing pyromellitic acid which comprises oxidizing polyalkyl-substituted aromatic aldehyde or oxidation derivative thereof in water solvent with molecular oxygen in the presence of an iron/manganese/bromine catalyst.
In the production of pyromellitic acid by oxidation of durene, loss of acetic acid as a solvent occurs by its combustion. Thus, a process for producing pyromellitic acid without using acetic acid has been required.
The case where oxidation reaction is performed in water solvent has the defect that polyalkyl-substituted aromatic aldehyde as a raw material for oxidation is high-priced. Further, there are some problems that when oxidation is performed in acetic acid solvent, it is avoidable to apply batch wise.